The present invention relates to registration of plural image exposures formed on a photoreceptor by a plurality of Raster Output Scanning (ROS) systems and, more particularly, to a method and apparatus for registering the image exposures in the process direction of the photoreceptor to form registered color images in a single pass.
This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a rotating polygon to a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to an apparatus for providing registration of the beam in the process direction movement of the photoconductive member.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. This laser beam is modulated in conformance with the image information. The modulated beam is transmitted through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoconductive member in a fast scan (i.e. line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. Laser printers may operate in either a single pass or a multiple pass system.
In a single pass, process color system, three ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. In a multiple pass system, each image area on the photoreceptor surface must make at least three passes relative to the transverse scan line formed by the modulated laser beam generated by a ROS system. With either system, each image must be registered to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (slow scan or skew registration) and in direction perpendicular to the process registration (referred to as fast scan or transverse registration).
The present invention is directed towards a method and apparatus for registering the color images in the process direction only by controlling registration errors at the lead edge of the images.
FIG. 1 shows a prior art, single pass, ROS color printing system 8 having four ROS systems, 10, 12, 14, and 16. The system 8 includes a photoreceptor belt 18, driven in the process direction, indicated by the arrow 19. The length of the belt 18 is designed to accept an integral number of spaced image areas I.sub.1 -I.sub.n represented by dashed line rectangles in FIG. 1. Upstream of each image area is a charging station (not shown) which places a predetermined electrical charge on the surface of belt 18. As each of the image areas I.sub.1 -I.sub.n reaches a transverse line of scan, represented by lines 20a-20d, the area is progressively exposed on closely spaced transverse raster lines 22, shown with exaggerated longitudinal spacing on the image area I.sub.4 in FIG. 1. Each image area I.sub.1 -I.sub.n is exposed successively by ROS systems 10, 12, 14, 16. Downstream from each exposure station, a development station (not shown) develops the latent image formed in the preceding image area. A fully developed color image is then transferred to an output sheet. Details of charge and development xerographic stations in a multiple exposure single pass system are disclosed, for example, in U.S. Pat. No. 4,660,059, commonly assigned as the present application and hereby incorporated by reference. The charge, development, and transfer stations are conventional in the art.
Each ROS system contains its own conventional scanning components, of which only two, the laser light source and the rotating polygon, are shown. The particular system 10 has a gas, or preferably, laser diode 10a, whose output is modulated by signals from control circuit 30 and optically processed to impinge on the facets of rotating polygon 10b. Each facet reflects the modulated incident laser beam as a scan line, which is focused at the photoreceptor surface. Control circuit 30 contains the circuit and logic modules which respond to input video data signals and other control and timing signals to operate the photoreceptor drive synchronously with the image exposure and to control the rotation of the polygon 10b by a motor (not shown). The other ROS systems 12, 14, 16, have their own associated laser diodes 12a, 14a, 16a, and polygons 12b, 14b, 16b, respectively. In the system of FIG. 1, transverse alignment of each successive image exposure is obtained, for example, by providing horseshoe shaped sensors 36a, 36b, 36c, 36d, which cooperate with optical targets T1, T2, T3, T4, respectively, formed in the belt surface. Other sensor systems could be used. Further details regarding transverse alignment registration are described in U.S. Pat. No. 5,208,796, commonly assigned as the present application and hereby incorporated by reference. However, for this prior art system, a process alignment must also be accomplished to ensure complete registration of the multiple image exposures.
Printing systems utilizing a ROS to form images on a photoreceptor surface are well known in the art. Conventionally, as seen in prior art FIG. 1, the ROS includes a diode or gas laser for generating a coherent beam of radiation; a modulator for modulating the laser output in accordance with an input video signal; and a multifaceted polygon scanner for scanning the modulated laser beam output line by line, across the surface of the photoreceptor to form the latent image. Also included in the ROS are various optical components to collimate, expand, focus and align the modulated scanning beams. These optical components are fixedly mounted within a housing frame, which is positioned within a printer machine frame, so that the modulated and shaped scanning beams emerging from an output window in the housing are directed in a scan line which is perpendicular to the photoreceptor surface. The lines will be formed in parallel across the surface of the photoreceptor belt.
Referring to FIG. 1, if the images I.sub.2, I.sub.3, I.sub.4 are to be perfectly registered with image I.sub.1, the leading edges should be parallel to each other. Each ROS system must be individually aligned to correct for the initial misregistration.
One solution to error while providing slow scan registration is to rotate an optical component of the ROS, as taught in U.S. Pat. No. 5,208,796, commonly assigned as the present application and hereby incorporated by reference.
One of the later optical components, such as one of the f.THETA. lens or the wobble correction mirror of the ROS is rotated to create the required rotation of the projected scan line. However, the lenses and mirrors of the ROS have optical power in at least one direction, usually both directions. Some optical elements, such as toroids, have optical power that vary with position of the beam on the surface of the element. Furthermore, a ROS is a precision optical system. With the obvious exception of the rotating polygon mirror, the optical components of the ROS should ideally be stationary and fixed. And these optical components are in the focal plane of the ROS.